Yeah, I think we would be asking for trouble with any of the big players from the east. It's pretty much going to come down to numbers with that lot, so all those countries that could mobilize huge numbers of ground troops.
And of course, we only have a professional Army, so we'd have to be pretty sharpish with conscription. And we lack the advantage that America has with every one of those bastards owning a gun.
So, countries I would NOT like to see knocking on our door with big guns:
China
Russia
USA (although the logistical task if they had no allies in Europe would make it tough for them)
Most of European countries really, they are all quite well trained and well equipped. Not that we would lose to many of the other Euros, but we'd lose our advantage with latest tanks and aircraft for example. And I like our Euro neigbours.
Countries I think we could take, BRING IT ON:
N.Korea, oh no, scary N.Koreans got 50 billion men, but I think we would royally fuçk them in the ass if they tried it bring it on our land. I'm not convinced of their military power in the air, sea and with the heavy stuff on land.
Any South Americans, central Americans etc.
Any of the other Asian countries.
All of Africa.
Yeah come on, who wants it!? But anyway, this is my uneducated opinion in this matter, someone will probably come and debunk my thoughts, but that's okay.